Abstract : This monograph examines the 1914 Marne and 1943 Kursk campaigns to determine, from the operational perspective, the reasons for decision failures and how intelligence influences them. Part I defines operational level intelligence, establishes the parameters of the study and selects decisions for discussion. It then presents the three reasons: lack of key data, incorrect analysis and ignoring intelligence and how they effect intelligence assessments. Part II, describes the two campaigns and discusses the operational decisions made by both belligerents. Part III, analyses the decisions to determine what intelligence was acceptance or rejection and the reasons for this. This monograph concludes that key data is usually available at the operational level of war. It further concludes that incorrect analysis of known data is important, however ignoring sound intelligence leads to the greatest number of decision failures.